Kosai (1163 - May 27, 1247) was a priest of the Jodo sect early in the Kamakura period. His bogo (assumed name) was Jokakubo.
Initially, he was a priest of Saito (western section) of Enryaku-ji Temple calling himself Shokabo Sho (鐘下房少輔), and he studied Tendai doctrine there. In 1198, he secluded himself from the world and became a follower of Honen.
In the 'Shichikajo seikai,' (seven articles of religious commandments) of 1204, he made his signature as the 15th person. Also, in 1206, when Kofuku-ji Temple appealed to In (the retired emperor), his name was pointed out, showing that he was energetically engaged in activities as a follower of Honen.
As a result, in 1207, when Honen was forced into exile to Tosa in the Kenei no honan (Kenei Persecution), he became exiled to Awa Province. However, he is said to have been saved from the exile, being held over to Jien. Further, in the so-called Karoku no honan (Karoku Persecution), which happened in 1227 after the death of Honan, his name was changed to 枝重 and he was exiled to Iki Province. As for this, it has also been said that he sent his disciple to Iki and that Kosai himself stayed in Sanuki Province. He was later forgiven, and it is said that he propagated nenbutsu (Buddhist invocation) at Kurihara, Shimofusa Province.
Kosai asserted ichinengi, which states that ojo (birth in the Pure Land) will be made just by making one shomyo (Invocation of the Buddha's name) or having one moment's belief.